Swirl jet burner

ABSTRACT

A burner includes a combustor having a generally cylindrical, hollow shape extending along a centerline, and a closed end. A fuel tube that is connectable to a fuel supply has at least one opening fluidly connecting an interior of the combustor with the fuel source such that fuel can be delivered from the fuel source into the interior of the combustor. At least one air inlet disposed to provide an air stream into the interior of the combustor. The air stream is arranged and configured to create a vortex flow structure within the combustor.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/063,037, which was filed on Oct. 13, 2014, thecontents of which are hereby incorporated herein in their entirety bythis reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to burners and, more specifically, toburners having a combustor.

BACKGROUND OF THE DISCLOSURE

Industrial furnaces operate at high temperatures, typically in the rangeof about 2400-3000° F., to promote furnace and process thermalefficiency. As a result, furnace and flame temperatures tend to be high,resulting in the generation of significant amounts of NOx emissions.

Efforts to control NOx emissions have resulted in the development ofvarious NOx control technologies. Such technologies inhibit NOxformation by modifying flame stoichiometry and the overall combustionprocess. Exemplary NOx control technologies include oxygen-enriched airstaging, in which oxygen-enriched air is introduced in stages into thecombustion process, exhaust gas recirculation, in which exhaust gas isintroduced into the primary combustion zone to reduce the flametemperature, fuel staging, in which the fuel is introduced in stagesinto the combustion process, and other methods such as oscillating andpulsed combustion. Although some of these controls have been partiallyeffective at controlling NOx emissions, they may not sufficientlyaddress NOx formation and reduction at the burner.

BRIEF SUMMARY OF THE DISCLOSURE

The present invention relates to flame stabilization in a burner using adecaying swirling air flow created by tangential jets in an annuluscombustor, which in the illustrated embodiments is formed between anouter shell and a gas pipe. A swirling motion imparted into air or anair/fuel mixture passing through the burner and provided to a burneroutlet is used to form flue gas recirculation (FGR) zones in thevicinity of the flame, which leads to a lower flame temperature and alower rate of nitrogen oxide (NOx) formation. Gas that is radiallyinjected into the axial-tangential air stream may further maximizeair/fuel mixing and increase combustion efficiency.

In the disclosed embodiments, a wide range of swirl numbers can beachieved at the burner outlet. The swirl number is controlled bychanging the number of tangential jets, the diameter of the jets, theincoming flow angle, or the length of the annulus, independently. Inthese respects, numerical simulations and experimental investigationshave shown that reducing the air flow rate results in slightly lowerswirl number while maintaining the burner stability at lower inputs,which results in very high turn-down ratios.

In one aspect, therefore, the disclosure describes a burner. The burnerincludes a combustor having a generally cylindrical, hollow shapeextending along a centerline, and a closed end. A fuel tube that isconnectable to a fuel supply has at least one opening fluidly connectingan interior of the combustor with the fuel source such that fuel can bedelivered from the fuel source into the interior of the combustor. Atleast one air inlet disposed to provide an air stream into the interiorof the combustor. The air stream is arranged and configured to create avortex flow structure within the combustor.

In another aspect, the disclosure describes a method for operating aburner. The method includes providing a combustor having a generallyhollow cylindrical shape defining a closed end and an open end, thecombustor defining an interior volume therein. The method furtherincludes injecting at least one stream of air into the interior volumein a direction that is tangential to a circular cross section of thecombustor adjacent the closed end, and inducing a swirling flow of theat least one stream of air into the combustor. A flow of fuel isprovided into the combustor such that the flow of fuel mixes with theswirling flow of the at least one stream of air to form a swirling flowof a combustible mixture, and the swirling flow of the combustiblemixture is expelled from the open end of the combustor. When theswirling flow of the combustible mixture is burning, at least a portionof combustion products created by the combustible mixture is admittedback into the swirling flow of the combustible mixture as thecombustible mixture is expelled from the combustor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic representations of a burner in accordancewith the disclosure.

FIGS. 2A and 2B are schematic representations of an alternativeembodiment for a burner in accordance with the disclosure.

FIGS. 3A-3D are outline and fragmentary views of a burner in accordancewith the disclosure.

FIG. 4 is a graphical representation of the effect of burner length onNOx emissions in a burner in accordance with the disclosure.

FIG. 5 is an outline view of a burner arranged in a shroud in accordancewith the disclosure.

FIGS. 6 and 7 are graphical representations of swirl number distributionin an annulus in two burner embodiments in accordance with thedisclosure.

FIGS. 8A and 8B are graphical representations of velocity fields withintwo burner embodiments in accordance with the disclosure.

FIG. 9 is a graphical representation of swirl number along therespective length of two burner embodiments in accordance with thedisclosure.

FIG. 10 is a graphical representation of swirl number with respect to arespective axial distance along two burner embodiments in accordancewith the disclosure.

FIG. 11A is an outline view of an alternative embodiment of a burnerarranged in accordance with the disclosure.

FIG. 11B is a fragmented view of the burner shown in FIG. 11A.

FIG. 12 is an outline view of a burner arranged with a manifold inaccordance with the disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a burner design that creates a stableswirling of air or an air/fuel mixture. The burners disclosed hereinrely primarily on the geometry of a combustor of the burner to achievethe desired flame characteristics. Advantageously, the disclosedcombustor geometries are simpler than existing designs and require nomoving parts. In other words, the burners described herein have beendeveloped to use air and/or fuel swirling to obtain better mixing thanexisting burners such that burner efficiency can be improved, and theeffect of FGR to minimize NOx emissions can be maximized.

A schematic representation for a design of a burner 100 is shown inFIGS. 1A and 1B. The burner 100 includes an air lance or air inlet 102and a gas or fuel inlet 104, which are provided to a generally elongate,cylindrical combustor 106 having an outlet opening 108 at onelongitudinal end. As shown, the combustor 106 has a tubular shape havinga closed end 110 opposite the outlet opening 108. The combustor 106 hasa frusto-conical shape or wall 112 that interconnects a combustor wallhaving a diameter D₀ to the outlet opening having a smaller diameter,D₁.

For creating a geometry in which air and fuel meet and mix withtangential momentum, and for creating low pressure regions (vortexes orvortex flow structures) to take advantage of internal flue gasrecirculation near the outlet of the combustor, where combustion occurs,the air and fuel inlets 102 and 104 are provided at an angle relative tocenterline 114 of the combustor 106. More specifically, the air inlet102 is disposed at an acute angle, θ₁, relative to the centerline 114,and at an axial location, l_(l), along the centerline 114 of thecombustor such that air entering the combustor 106 carries momentumcomponents in both the radial and axial directions with respect to thecenterline 114. Similarly, the fuel inlet 104 is disposed at an acuteangle, θ₂, relative to the centerline 114, and at an axial location, l₂,along the centerline 114 of the combustor such that fuel entering thecombustor 106 also carries momentum components in both the radial andaxial directions with respect to the centerline 114. The angles andaxial locations of the air and fuel inlets 102 and 104 may be the sameor different. In the burner embodiment shown in FIG. 1B, the air andfuel inlets are disposed tangentially with respect to the circular outerwall of the combustor 106.

Geometrical parameters contributing to fuel/air mixing and to creationof recirculation zones have been determined to include the distance fromthe end of the combustor to the air and gas jets, respectively, l₁ andl₂, the air and fuel injection angles, respectively, θ₁ and θ₂, and thecombustor and outlet opening diameters, respectively, D₀ and D₁. Otherfactors that may affect burner performance include the achievableturn-down ratio of the burner. As used herein, “turn-down ratio” refersto the ratio of maximum heat output that can be achieved by the burnerto the minimum level of heat output at which the burner will operateefficiently or controllably.

An alternative embodiment for a burner 200 is shown in FIGS. 2A and 2B.The burner 200 includes an air inlet 202 and a fuel inlet 204. In thisembodiment, the air inlet 202 is provided tangentially relative to agenerally elongate, cylindrical combustor 206. Fuel is provided to thecombustor 206 through a fuel tube 204, which is concentrically disposedwithin the combustor 206 and extends along a combustor centerline 214from a closed end 210 to an outlet opening 208. The combustor 106 has acylindrical shape having a diameter D₁, which is larger than an outletdiameter D₂ of the fuel tube 204. The relative position and dimensionsof the combustor 106 and fuel tube 204 create a generally cylindrical,annular space 216 extending axially along the combustor 206 and radiallybetween the outer shell of the combustor 106 and the fuel tube 204.

During operation, air provided to the combustor 206 swirls as it travelsalong the combustor 206 within the annular space 216. The swirling airin the annular space 216 mixes with fuel exiting the fuel tube 204 atthe outlet 208 to form a combustible mixture that forms a flame. Theswirling flow of air in the annular space 216 is created when atangential velocity component is added to the velocity vector of airentering the combustor 206 through the tangential air inlet 202.

For swirling jets, different flow regimes may be identified depending onthe degree of swirl present in the jet. The term “swirl number” is knownin the art and is used herein to refer to a ratio of the maximumtangential velocity to the axial velocity of a flow. Thus, the swirlnumber is a non-dimensional parameter that is indicative of the amountor extent of swirling within a flow. In the burner 200, for low swirlnumbers (i.e. when the maximum tangential velocity is of the order of50% or less of the axial centerline velocity), the air flow or jetbehaves in a similar way as for the non-swirling case, even though somemodification in the mean and fluctuating velocity distributions, jetwidth or spreading are present. However, when the swirl becomes strong(i.e. when the tangential velocity becomes larger than the axialvelocity), formation of a recirculation zone may result in a reverseflow. This phenomenon is usually referred to as a vortex breakdown flowregime. For creating a vortex breakdown regime downstream of a pipe,i.e., after the outlet of the burner, tangential air jets are injectedinto the annular space 216 of the combustor 206.

In general, higher swirl numbers will result in a more expanded flame atthe outlet 208, which in turn increases a zone of recirculation forcombustion products to be entrained within the flame near the outlet ofa combustor. Flue gas recirculation (FGR) lowers flame temperature and,thus, NOx production. However, excessive FGR entrainment mayde-stabilize combustion and even cause flame blow-out. On these bases, arange of swirl numbers for a given burner configuration at which NOxemissions can be sufficiently reduced while maintaining flame stabilitycan be determined and implemented for a particular burner configuration.

An alternative embodiment for a burner 300 is shown in FIGS. 3A-3D. Theburner 300 includes four air inlets 302 arranged tangentially around aclosed-end cross section 310 of a combustor 306. The combustor 306includes a converging, frusto-conical outlet 309. A fuel inlet devicefor providing fuel to the interior of the combustor 306 is embodied inthe illustrated embodiment as a fuel tube 304. The fuel tube 304 isconcentrically disposed within the combustor 306 and extends along acombustor centerline 314 from a closed end 310 to an outlet opening 308.The combustor 306 has a cylindrical shape having a diameter D₁, which islarger than an outlet diameter D₂ of the fuel tube 304. The relativeposition and dimensions of the combustor 306 and fuel tube 304 create agenerally cylindrical, annular space 305 extending axially along thecombustor 306 and radially between the outer shell of the combustor 306and the fuel tube 304.

During operation, air provided to the combustor 306 swirls as it travelsalong the combustor 306 within an annular space 305 formed between thewalls of the combustor 306 and the fuel tube 304. The swirling air inthe annular space 305 mixes with fuel exiting the fuel tube 304 at anoutlet opening 308 of the combustor 306 to form a combustible mixturethat, when ignited, produces a flame. The swirling flow of air in theannular space 305 is created when a tangential velocity component isadded to the velocity vector of air entering the combustor 306 throughthe tangential air inlets 302. In the illustrated embodiment, four pipesform the air inlets 302. The air inlets 302 are tangentially connectedto the end of the combustor 306. The fuel tube 304 is formed as aclosed-end tube having its closed end disposed at least partially withinthe combustor 306 along the centerline 314. A number of holes 316 aredrilled in the wall of the fuel tube 304 to permit gas or another fuelto enter the internal space of the combustor 306 and mix with the airtravelling therein.

The number, size and location of the holes 316 is adjusted and selectedto provide a desired gas pressure. As shown, the holes 316 are disposedsymmetrically around a cross section of the fuel tube 304 at an axiallocation that is selected. Alternatively, the axial location,orientation, and arrangement of the holes can be different than thearrangement shown. The plane on which the injection holes are locatedmay fall anywhere on the gas pipe, and there may be multiple planes,e.g., there may be holes at both the start and end of the gas pipe. Theoptimum gas pressure will maximize the effect of mixing time and gaspenetration in the air stream. For example, for the burner shown inFIGS. 3A-3D, sixteen 2 mm-holes were found to minimize thermal NOxemissions while also producing a gas pressure drop of 14 inches ofwater.

A graph 400 showing the effect of the outlet swirl number on thermal NOxformation is shown in FIG. 4. For conducting the experiment, a burnersimilar to the burner 300 (FIG. 3D) without a converging outlet 309 wasconstructed and tested with a relatively long combustor (1.5 m). NOxemissions were measured for various combustor lengths. The obtained datais shown in FIG. 4, where burner length (in meters) is expressed alongthe horizontal axis and NOx concentration (in g/L) is expressed in thevertical axis 403. Two curves were produced, each representing differentcombustion mixtures. A first curve 402 represents an experiment where37% excess air is provided relative to stoichiometric combustionmixtures. A second curve 404 represents 55% excess air. As can be seenfrom the graph 400, a correlation exists in which longer burner lengthcontributes to an increase in NOx emissions. For lengths shorter than0.4 meters, for the particular combustor diameter tested, the flame wasextinguished. It is believed that as the combustor length decreases from1.5 m to 0.4 m, NOx production is lowered significantly as a result ofhigher swirl number and more FGR associated with shorter combustor. Forcombustor lengths shorter than 0.4 m, excessive FGR extinguishes theflame and the burner becomes unstable.

The role of the converging cone 309 (FIG. 3) at the end of the combustoris also notable. It is contemplated that reducing the outlet diameter(as a result of the cone) of the combustor decreases the outlet swirlnumber and increases the outlet velocity of the fuel/air mixture exitingthe combustor 306 through the outlet opening 308. The increased outletvelocity shifts the flame anchorage location to a position ahead of thegas injection holes 316. For illustration, it is noted that without thecone, the flame is anchored at the fuel injection holes. This means thatmixing time is increased and the flame is more exposed to the FGR. Also,lower swirl number helps the stability of the burner and higher outletvelocity prevents flash-back.

With the converging cone 309, the burner can operate in a semi-premixedcondition. Semi-premixed condition, as used here, means that only aportion of the fuel required for stoichiometric combustion can beprovided through the fuel tube disposed in the combustor. The remainingfuel can be provided to the active flame at the end of the combustor. Inone test that was conducted, gas was injected into the combustor from agap or opening in the gas line inside the combustor (far from theoutlet) and it was observed that, while the gas pressure wassignificantly lowered (the gap was much bigger than the nozzle holes),NOx emissions and the flame anchor location were unaffected. The coneangle, as measured by an acute angle extending from the outer peripheryof the combustor towards the centerline, can range between 0 and 90degrees. As shown, the angle is about 13 degrees. On the basis of theexperiment, we determined that the flame dynamics, stability andemissions are all strong functions of the size and the location of therecirculation zone created as a result of the swirling air flow, andequally the swirl number near the combustor outlet.

To lower carbon monoxide emissions, the burner 300 may be operated inconjunction with a shroud 500, as shown in FIG. 5. The shroud 500 may beadded to the outside of the burner 300 to help lower carbon monoxidefrom the burner. In the illustrated embodiment, the shroud 500 has acylindrical shape that is open on both ends and is concentricallydisposed relative to the combustor 306. A diameter of the shroud 500 islarger than the diameter of the combustor 306 to form an annular volume502 therebetween. During operation, exhaust gas produced by combustionis at least temporarily enclosed within the annular volume to provide anopportunity for carbon monoxide (CO) present therein to further oxidizeinto carbon dioxide (CO₂). In the illustrated embodiment, while use ofthe shroud 500 may increase NOx generation due to increased dwell timeof combustion gases in the annular volume 502, and also an increasedflame temperature, but CO emissions are reduced by the further oxidationof CO into CO₂ such that a balance may be selectively controlled betweenNOx and CO production by adjusting the dimensions and other structuralfeatures of the shroud 500, for example, the diameter and/or length ofthe shroud. As the length of the shroud increases the temperature andresidence time of the hot flue gases increases, which will cause areduction in CO emissions and an increase in NOx emissions. Similarly,as the diameter of the shroud increases the temperature of the fluegases decreases, increasing CO emissions and reducing NOx emissions.

For NOx creation to remain at the desired low rate, portions of theshroud may be left open 1100. In an embodiment of the burner, theshroud's open area is created by making holes in the shroud upstream ofthe base of the flame. The open areas in the shroud allow for flue gasesto be drawn into the flame by the swirling flow and reduce flametemperature at the base of the flame where a significant portion of thetotal NOx from the burner is created. During operation the flue gases atthe base of the flame reduce peak flame temperature to abate NOxcreation, while the shroud creates an enclosure that promotes theoxidation of carbon monoxide into carbon dioxide. In one exemplaryembodiment, the low NOx emissions of the burner without a shroud, under20 ppm NOx, are combined with the low CO emissions of the burner with ashroud, down to 0 ppm CO, while firing with 15% excess air. Flamestability was improved with a shroud as the total volume of flue gasrecirculation (FGR), which describes the portion of flue gas that isrecirculated into the flame, can be reduced for equivalent NOxproduction as the FGR that is drawn into the flame is directed to thebase of the flame where it has the most effect on NOx production. Theshroud also creates a high temperature zone around the flame that isabove the auto-ignition temperature, which continually reignites theflame making it very stable. The amount of FGR drawn into the flame canbe precisely controlled by the location of the open area and size of theopen area in the shroud for specific firing environments. In at leastone embodiment of the burner, the flow area of holes formed in theshroud are externally adjustable, for example, by a gate or other typeof valve, to dynamically control the amount of FGR drawn into and/oraround the flame.

Various additional experiments were conducted to determine theparameters affecting burner operation and efficiency. For example, toinvestigate the swirl number distribution along the length of thecombustor and understand the effect of the converging cone on the swirlnumber, numerical modeling was performed on annuluses with and without aconverging cone. The results are shown in graphical form in FIGS. 6 and7. As can be seen in FIG. 6, the swirl number, which is arranged on thevertical axis, exponentially decreases along the length of the combustoras the distance from the air inlets (e.g., air inlets 302) increases, asrepresented by the horizontal axis. When a converging cone (e.g., 309)is added, swirl behavior changes and the outlet swirl number decreaseswhen all other structures of the burner are maintained the same as aburner having the same structure but without a converging cone. Theeffect of the converging cone was also tested to determine its effect onNOx emissions, which were unexpectedly found to be 50% lower when theconverging cone was added to the combustor. An additional unexpectedeffect of adding a converging cone was an improvement in the turn-downratio, which improved from 10:1 for a straight combustor to higher than40:1 for a combustor with a converging cone.

To further investigate the effectiveness of the converging cone on theperformance of the burner, the length of the burner was furthershortened to examine burner stability using CFD simulations and physicaltesting. The results show that by gradually reducing the outlet diameterusing the converging cone, the burner remains stable, i.e., without aninward gas stream, as the outlet swirl number remains substantiallyunchanged. A CFD simulation of velocity profiles within the combustorare shown in FIGS. 8A and 8B, where the velocity field is depicted for0.4 m and 0.2 m long combustors, not including the length of theconverging cone, which is about 75 cm. The swirl decay for the burnerswith different lengths as shown in FIGS. 8 A and 8B stabilizes after agiven distance such that, at the outlet of each burner, at theconverging cone, the swirl numbers equalize. FIG. 9 is a plot of thesame swirl numbers over a normalized burner length.

To determine the effect of the number of lances (air inlets) on theperformance of the burner, simulations and experiments were performed tocompare burners with two and four lances. The results of thisinvestigation are shown in the graph of FIG. 10, in which swirl numberis plotted on the vertical axis against burner length, which is plottedagainst the horizontal axis. Data points acquired are plotted as curves.A first curve 902 represents information acquired for a burner havingtwo lances (air inlets), and a second curve 904 represents informationacquired for a burner having four lances (air inlets). In both burners,the diameter of the air inlets was scaled to maintain substantiallyuniform air velocity at the outlet opening of the burner. As can be seenin FIG. 10, the outlet swirl number is preserved and, therefore, theburner stability and NOx emissions are also expected to be maintained,as discussed above. On the basis of these experiments, it iscontemplated that burner dimensions can be arranged to preserve adesired swirl number with a single lance (air inlet).

It has been determined that an even or generally symmetricaldistribution of lances around the burner may promote efficientoperation. As shown in FIGS. 11A and 11B, an alternative embodiment fora burner 1000 includes an air distribution manifold 1002, a crosssection of which is shown in FIG. 11B. Similar to the burner 300 (FIGS.3A-3D), the burner 1000 includes a cylindrical combustor 106, which inthis embodiment is constructed from two segments, a base segment 1004and an end segment 1006 that also forms the outlet opening 308 and thefrusto-conical outlet 309. A pair of flanges 1008 connects the base andend segments 1004 and 1006, which segments can be made from differentmaterials such as mild steel for the base segment 1004 and stainlesssteel for the end segment 1006.

The distribution manifold 1002 in the illustrated embodiment fluidlyconnects a single air inlet opening 1010 to two lance outlet openings1012. Air to the air inlet opening 1010 may be provided during operationfrom a blower or other air source. Each of the two lance outlet openings1012 is formed at an end of a, respective, lance air passage 1014 havinga major axis 1016 that is disposed generally tangentially at an offsetrelative to an inner diameter, D, of the combustor 306. An annularpassage 1018 extends peripherally around a portion of the combustor 306to fluidly interconnect the single air opening 1010 with the two lanceair passages 1014. The general shape of the annular passage 1018, andalso the placement of the lance air passages 1014 along the annularpassage 1018, can be selectively arranged to provide a balanced, evenflow of air from the air opening 1010 through each of the two lanceoutlet openings 1012.

In the illustrated embodiment, the manifold 1002 is constructed bythin-wall sheet metal and is structured to include two concentriccylinders with face and back plates to enclose a hollow cylindricalplenum into which the single air inlet and two lance outlets are formed.The manifold is designed such that the burner body is set concentricallyin the manifold and the lances of the burner are located at themid-plane between the back plate and face plate of the manifold. Thelance outlets 1012 of the manifold are located such that they are set180° from each other with one being between 70°-80° from the inlet andthe other 100°-110° from the inlet, but other angles can be used. In oneembodiment these angles are 75° and 105°, respectively. From acombination of physical and analytical testing, for example, usingcomputational fluid dynamic (CFD) models, these angle ranges weredetermined to lead to approximately equal flow and pressure drop througheach lance.

An alternative embodiment of a shroud 1020 disposed around the burner1000 is shown in FIG. 12. In this illustration, a shroud 1022 isdisposed around an open end of the combustor 306, similar to the shroud500 shown in FIG. 5, but in this embodiment, the shroud 1022 includes aplurality of windows 1024 formed along a periphery of the shroud 1020 ingeneral axial alignment around the outlet opening 308. Each of thewindows 1024 has a generally rectangular shape, with rounded corners,and permits fluids to pass therethrough during operation. An open areaof each window 1024 that is available for fluid flow can be fixed, basedon the size and dimension of each window, or can be adjustable, as is inthe embodiment shown in FIG. 12. In this embodiment, a collar 1026having a hollow cylindrical shape is slidably disposed along an outersurface of the shroud 1022. The collar 1026 has a leading edge 1028 thatmoves, as the collar moves, towards or away from the burner 1000,manually or by action of an actuator 1030 to selectively cover a portionof the flow area provided by the windows 1024, thus effectivelycontrolling the flow area available for fluid flow through the shroud1022 depending on the operating conditions of the burner 1000. Controlof the opening of the windows 1024 by the actuator 1030 may be carriedout automatically based on the operating conditions of the burner, forexample, flame temperature, fuel flow rate, air temperature, emissions,and the like.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

We claim:
 1. A burner, comprising: a combustor having a generallycylindrical, hollow shape extending along a centerline, and a closedend; a fuel tube that is connectable to a fuel source and having atleast one opening fluidly connecting an interior of the combustor withthe fuel source such that fuel can be delivered from the fuel sourceinto the interior of the combustor; at least one air inlet disposed toprovide an air stream into the interior of the combustor, the air streamarranged and configured to create a vortex flow structure within thecombustor.
 2. The burner of claim 1, wherein the combustor is configuredto maintain the vortex flow structure through a length of the combustoralong the centerline such that fuel from the fuel tube is mixed with theair stream, and to deliver a swirling mixture of air and fuel from anopen end of the combustor.
 3. The burner of claim 1, further comprisinga converging nozzle disposed at an open end of the combustor oppositethe closed end.
 4. The burner of claim 1, wherein the at least one airinlet is disposed to inject the air stream in a tangential directionrelative to a circular cross section of the combustor adjacent theclosed end, such that the air stream has momentum components in both aradial direction and an axial direction with respect to the centerline.5. The burner of claim 1, further comprising an additional air inletproviding an additional air stream into the combustor.
 6. The burner ofclaim 1, wherein the fuel tube is disposed in the combustor and extendsgenerally concentrically along the combustor.
 7. The burner of claim 1,wherein the fuel tube is disposed to inject fuel into the combustor in atangential direction with respect to a circular cross section of thecombustor.
 8. The burner of claim 7, wherein the air stream and the fuelare provided in opposing directions into the interior of the combustor.9. The burner of claim 1, further comprising an air distributionmanifold having an air inlet opening in fluid communication with an airdistribution passage, the air distribution passage being fluidly open tothe at least one air inlet.
 10. The burner of claim 9, wherein the airdistribution manifold has a generally hollow cylindrical shape disposedaround the combustor adjacent the closed end, the air distributionmanifold forming internally an annular passage extending around thecombustor and surrounding a central ring, and wherein the at least oneair inlet is formed as a straight passage extending through the centralring between the annular passage and the interior of the combustor. 11.The burner of claim 10, wherein a second straight passage defining asecond air inlet is formed in the central ring, the second straightpassage being oriented in parallel with the at least one air inlet anddisposed at an offset therewith such that the at least one air inlet andthe second straight passage are both opposingly tangential at an offsetrelative to a circular cross section of the combustor.
 12. The burner ofclaim 11, wherein the at least one air inlet is disposed between 70°-80°with respect to the air inlet opening and the second straight passage isdisposed between 100°-110° with respect to the air inlet opening arounda periphery of the combustor.
 13. The burner of claim 1, furthercomprising a shroud disposed around an open end of the combustoropposite the closed end.
 14. The burner of claim 13, wherein the shroudhas a generally cylindrical shape and forms a plurality of openings thatare axially aligned with the open end of the combustor.
 15. The burnerof claim 14, further comprising a collar being slidably selectivelydisposed along the shroud, wherein the collar is configured to cover atleast partially each of the plurality of openings.
 16. The burner ofclaim 1, wherein the burner includes one or two pairs of opposed airinlets, each pair of air inlets being disposed in parallel and at anoffset distance such that each air inlet provides the air stream in atangential direction with respect to a circular cross section of thecombustor.
 17. The burner of claim 1, wherein the combustor forms anopen end opposite the closed end, the open end including an outletopening formed at an end of a converging nozzle.
 18. A method foroperating a burner, comprising: providing a combustor having a generallyhollow cylindrical shape defining a closed end and an open end, thecombustor defining an interior volume therein; injecting at least onestream of air into the interior volume in a direction that is tangentialto a circular cross section of the combustor adjacent the closed end;inducing a swirling flow of the at least one stream of air into thecombustor; providing a flow of fuel into the combustor such that theflow of fuel mixes with the swirling flow of the at least one stream ofair to form a swirling flow of a combustible mixture; expelling theswirling flow of the combustible mixture from the open end of thecombustor; and when the swirling flow of the combustible mixture isburning, admitting at least a portion of combustion products created bythe combustible mixture back into the swirling flow of the combustiblemixture as the combustible mixture is expelled from the combustor. 19.The method of claim 18, wherein the flow of fuel is provided through afuel tube extending along the interior volume of the combustor.
 20. Themethod of claim 18, wherein the flow of fuel is provided as a streamtangentially into the interior volume with respect to the circular crosssection of the combustor adjacent the closed end.